A massive United Nations emergency relief operation is underway to provide lifesaving assistance to millions of victims of Nepal’s devastating earthquake. The U.N. says the search and rescue phase of the operation is winding down and moving toward the recovery phase.

Nepal’s 7.8-magnitude earthquake is estimated to have killed more than 4,600 people and injured more than 9,000. The numbers are expected to rise. Some 8 million people are affected by the disaster. Damage to infrastructure, residential buildings and unique heritage sites is huge and still to be fully assessed.

The U.N. Office for the Coordination of Humanitarian Affairs is coordinating the relief operation that has yet to hit its stride. International and national rescue teams have been searching for survivors since Saturday. OCHA spokesman Jens Laerke says unfortunately, the window for rescuing people is narrowing.

“This is a race against time," said Laerke. "It is also a race against a moving target in the sense that we still do not have a full assessment of the needs and requirements in the rural areas outside of Kathmandu. As you know, the disaster itself has created a lot of infrastructure problems and it is simply a country that, because of its geography, it is such that it is difficult to move into these areas.”

The United Nations has released $15 million dollars from its emergency fund to help humanitarian aid organizations rapidly scale up operations and provide immediate assistance to people in desperate need. The U.N. plans a flash appeal in the coming days to pay for more comprehensive, long-term needs.

The World Health Organization reports an emergency operation center run by the government in partnership with aid agencies is in full swing. It says some hospitals have been damaged in the earthquake, but they are functional, albeit at a low level.

The WHO director of operations for emergencies, Richard Brennan, says major concerns are life-threatening injuries including head and spinal injuries and crush syndrome fractures.

“There are other health concerns that we have to be aware of as people are exposed with minimal shelter gathering in camp-type settings where we are concerned about the potential spread of infectious disease and are taking measures to address that," said Brennan. "We have to make sure that women who continue to give birth have access to obstetric services and we will be looking at the other longer term health issues, such as psycho-social and impact on access to care for chronic medicines over time.”

Brennan says WHO has distributed medical supplies for 120,000 people for three months.

Many people are living in makeshift camps and open areas in the rain as their homes have been damaged or because they fear aftershocks. Shelter materials are urgently needed. The U.N. refugee agency on Tuesday airlifted relief supplies to Kathmandu containing plastic sheets and solar lamps for 30,000 quake victims. It previously had sent five truckloads of similar goods for more than 40,000 survivors in three districts east of the capital.

The World Food Program is gearing up to provide food for 1.4 million people over the next three months. The U.N. Children’s Fund estimates 1.3 million children are in need of urgent humanitarian assistance. The agency is preparing to airlift essential items including medical supplies and water purification tablets to stave off diseases from contaminated water.